1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method of forming an alignment layer in an LCD capable of a high quality picture.
2. Discussion of the Related Art
In general, a cathode ray tube (CRT) has been most widely used among display devices for displaying image information on a screen until recently, but the CRT has many inconveniences due to large volume and heavy weight as compared to the display area.
With the development of various electronic devices, display devices are now being used in personal computers, notebook computers, wireless terminals, vehicle instrument panels, electronic display boards, and the like. Also, due to the development of high speed information communication technology, it is possible to transmit large amounts of image information, and the importance of a next generation display device capable of processing and displaying the large amounts of image information is increasing.
Such next generation display devices are required to realize lighter, thinner, shorter and smaller displays, high luminance, large-sized screen, low power consumption, and a low price. Among such next generation display devices, a liquid crystal display device (LCD) are gaining wide use.
The LCD exhibits a greater resolution than other flat displays, and a rapid response time comparable to that of the CRT to implement a moving picture.
One of the LCDs that are widely used at the present time, is a twisted nematic (TN) mode LCD. In the TN mode LCD, including electrodes on two substrates and liquid crystal directors twisted by 90°, a driving voltage is applied to the electrodes to drive the liquid crystal directors.
However, the TN mode LCD has a serious drawback of a narrow viewing angle.
Recently, LCDs employing new modes of operation are being actively researched so as to solve the problem of the narrow viewing angle. Examples of these new modes of operation include an in-plane switching (IPS) mode, an optically compensated birefringence (OCB) mode LCD, etc.
The IPS mode LCD generates a horizontal electric field so as to drive the liquid crystal molecules in a horizontal state with respect to the substrates by forming two electrodes on one substrate and applying a voltage between the two electrodes. In other words, the longer axis of the liquid crystal molecule does not stand up with respect to the substrates but rotates horizontally.
As a result, the IPS mode LCD has a small variation in the birefringence of liquid crystal according to a visual direction and thus has an excellent viewing angle characteristic compared to the TN mode LCD.
FIG. 1 is a flowchart illustrating a method of fabricating a related art LCD.
Referring to FIG. 1, first and second substrates having a plurality of patterns formed thereon are first manufactured in operation S100.
The first (or bottom) substrate includes a matrix-type array device including a thin film transistor (TFT), and the second (or top) substrate includes a color filter.
In operation S110, the substrates are cleaned to remove a foreign substance thereon. In operation S120, a polyimide material is printed on the substrate to form an alignment layer.
In operation S130, the alignment layer is dried and hardened using heat.
In operation S140, a surface of the hardened alignment layer is rubbed in one direction.
In operation S150, an adhesive seal pattern is formed at an edge of the second substrate except at a region where a liquid crystal injection hole remains and spacers are dispersed on the first substrate.
In operation S160, the first and second substrates are attached together with an accuracy of several μm so as to prevent light leakage.
In operation S170, the attached substrates are cut into unit cells. This cutting process includes a scribing process forming lines on the first and second substrates and a breaking process for dividing the scribed substrate into unit cells.
In operation S180, liquid crystal is injected through an injection hole into a gap between the first and second substrates cut into cells and the injection hole is sealed to complete the LCD.
A liquid crystal dispensing method can be used instead of the liquid crystal injecting method. In the liquid crystal dispensing method, after liquid crystal is first dispensed between the first and second substrates, the first and second substrates are attached together.
The physical characteristics of the liquid crystal are changed as the molecular arrangement varies, and thus the arrangement varies in response to an external force such as an electric field.
Due to the characteristics of the liquid crystal molecule, a control technique for arranging the state of the liquid crystal molecule is essential for operation of the LCD.
Specifically, a rubbing process to uniformly align liquid crystal molecules in one direction is essential for the normal operation of the LCD and a uniform display characteristic thereof.
A related art alignment layer forming process for determining an initial alignment direction of liquid crystal molecules will now be described in detail.
The forming of an alignment layer includes a process of depositing a high polymer thin layer and a process of aligning an alignment layer in one direction.
The alignment layer is typically made of a polyimide series organic material and is typically aligned through a rubbing process.
A polyimide series organic material may be deposited on a substrate and a solvent thereof is volatilized at about 60-80° C. Thereafter, the deposited material is hardened at about 80-200° C. to form an alignment layer. The alignment layer is rubbed in one direction with a roller wound with a rubbing cloth such as velvet to form an alignment direction thereon.
This rubbing process enables an easy and stable alignment process, and is thus suitable for mass production of the LCD.
However, a defect in the rubbing operation occurs when the rubbing cloth becomes defective during the rubbing operation.
Because the rubbing process is performed through direct contact between the rubbing cloth and the alignment layer, the rubbing process may cause the contamination of a liquid crystal cell due to particles, the damage of a TFT due to an electrostatic discharge, the necessity of an additional cleaning process after the rubbing process, and/or a non-uniform alignment of liquid crystal in a wide-screen LCD, leading to degradation in production yields of the LCD.
FIGS. 2A and 2B are respectively a sectional view and a plan view illustrating an alignment state of liquid crystal around a step portion of an electrode pattern such as a pixel electrode and a common electrode in a related art LCD.
Recently, an improved IPS mode LCD for improving the viewing angle and an IPS mode LCD manufactured using 3-4 masks so as to reduce the number of manufacturing processes has been developed and used. In these IPS mode LCDs, the step difference of the stepped edge portion is increased that causes an increase in the number of alignment defects.
Referring to FIGS. 2A and 2B, an alignment layer 151 is formed on a pixel electrode 130 patterned on a first substrate and the pixel electrode 130 has a step edge portion with a predetermined step difference.
A color filter layer 160 and an alignment layer 152 are formed on a second substrate facing the first substrate and a liquid crystal layer 190 is formed between the second and first substrates.
A common electrode, a TFT region, a gate line, and a data line also have a stepped edge portion
The stepped edge portions of the electrode and line patterns in a pixel region cause a non-uniform alignment of liquid crystal in nearby regions.
If the liquid crystal is in a normally-black mode, a black color is displayed when no voltage is applied.
However, light leakage occurs in a region A shown in FIGS. 2A and 2B when no gate voltage is applied.
That is, when no voltage is applied, the liquid crystal must be aligned in the same direction as the rubbing direction of the alignment layers 151 and 152.
However, the stepped edge portion of the electrode causes a non-uniform liquid layer 191 having an alignment direction different than the rubbing direction and also causes the liquid crystal of a uniform liquid layer 192 to have an alignment direction different than the rubbing direction.
The non-uniform liquid crystal causes phase retardation of the light. The phase retardation causes a linearly-polarized light to change into an elliptically-polarized light. The elliptically-polarized light causes phase retardation in a uniform liquid crystal layer formed near the color filter layer 160, resulting in a large phase retardation.
Consequently, when no voltage is applied in a normally-black mode, light of the backlight assembly passes through the region A. This causes light leakage in a black display state and a decrease in a contrast ratio, thereby making it difficult to achieve a high image quality.
Recently, an improved IPS mode LCD for improving the viewing angle and an IPS mode LCD manufactured using 3-4 masks so as to reduce the number of manufacturing processes have been developed. In these IPS mode LCDs, the step difference of the stepped edge portion is increased to cause an increase in the alignment defect.
Accordingly, there remains a need for a device and method for preventing the LCD image degradation due to the stepped edge portion, such as increased black brightness and reduced contrast ratio.